Wednesday, 7 November 2012

The Uncertain Blitzkrieg Down Under

By the late Quartenary, Australia had lost 23 out of 24
genera of its megafauna. The 2 main explanations for this are human mediation –
overkill and habitat destruction – and climate change. In this blog post I will
look at each in turn, and conclude that humans are not the primary cause of
megafauna extinction in Australia. While human ‘blitzkrieg’ was previously the
consensus, new research has shown increasing evidence that it is likely climate
change played a larger role in a majority of extinctions.

Human Overkill and
Habitat Destruction

The main argument for proponents of the blitzkrieg
hypothesis is that most megafauna were present when humans arrived in
Australia, and they were subsequently wiped out by interaction with humans.
They point to the evidence that extinction of megafauna occurred around 45,000
calendar years ago, coinciding with human colonization of Australia and
predating climate change (Roberts et al 2001). Miller et al (2005) also show in
their research that records have shown a decline in food sources for the
Australian emu and marsupial wombat, attributed to human modification of the
drought- adapted environment of shrubs and grasses into the fire-adapted scrublands
of today.

However, Trueman et al (2005) argue that fossil evidence from
Cuddie Springs (Southeastern Australia) and other sites refute the consensus
that megafauna became extinct about 45,000 calendar years ago. Their research
has found that humans and megafauna co-existed for about 15,000 calendar years
after the arrival of humans. Besides, Wroe et al (2004) points to the complete
absence of any direct evidence implicating human mediation, e.g. kill sites, similar
to those that have been found in North America.

Climate Change

Wroe and Field (2006) point to evidence from a variety of
climate proxies (pollen, charcoal, hydrology, etc.) that there was a broad
trend towards increasing aridification of the Australian climate in the Late
Quarternary, which overlaid glacial cycles. Thus, the Holocene interglacial was
drier than previous interglacials, disputing the idea that the
Pleistocene-Holocene transition was similar to previous transitions. The chart
below shows that lake levels and river flow in Northern and Southeastern
Australia started declining from around 50,000 calendar years ago, while dust
levels increased from around 200,000 calendar years ago. Other evidence from
pollen records show that around 200,000 years ago, grasses became more
prevalent relative to eucalyptus, indicating increased aridity.

Fig. 1: Wroe and Field (2006)

Thus, non human-mediated climate change can be shown to have
caused the extinction, as these natural processes were in motion long before
humans arrived. However, it is certainly possible that humans played a role in
further stressing megafauna already stressed by these climatic changes,
although they cannot be seen to be a primary cause of the extinctions.

An Alternative Model:
Staggered Extinctions of Australian Pleistocene Megafauna

Wroe and Field (2006) suggest an alternative model of
staggered extinctions. Their more recent research shows that at least 65% of
the megafauna cannot be shown to have existed beyond 130,000 calendar years
ago. Only 13% of megafauna species during the Pleistocene co-existed with
humans, and at least half of these species survived 15,000 calendar years after
humans arrived. Thus, they argue for an alternative model of repeated range
contractions and limitation of refugia for megafauna as Australia’s climate
became increasingly arid, resulting in extinctions which predated human
contact.

Their model is supported by Cosgrove and Allen’s (2001)
study of Tasmanian rockshelters. Since early humans could only have reached
Tasmania by 37,000 calendar years ago through the development of a land bridge,
if humans caused extinction then megafauna should be shown to survive there
until the arrival of humans. However, no megafauna fossil remains younger than
46,000 calendar years ago could be found.

References

Cosgrove, R. and Allen, J. (2001) ‘Prey choice
and hunting strategies in the Late Pleistocene: evidence from Southwest
Tasmania’ in Lilley, A. and O’Conner, S.
(Eds.), Histories of Old Ages: Essays in Honour
of Rhys Jones. Canberra: Research School of Pacific and Asian Studies, Australian
National University, Canberra, pp. 397–430.

Roberts, R.G. et al (2001) New ages for the last Australian megafauna continent-wide extinction about 46,000 years ago. Science, 292, 1888–1892.

Trueman et al
(2005) ‘Prolonged coexistence of humans
and megafauna in Pleistocene Australia’, Proceedings
of the National Academy of Sciences of the United States of America, 102,
23, pp. 8381-8385

Wroe, S. et
al (2004) ‘Megafaunal extinction in the Late Quaternary and the global overkill
hypothesis’, Alcheringa, 28, pp. 291–331.Wroe, S.
and Field, J. (2006) ‘A review of the evidence for a human role in the
extinction of Australian
megafauna and an alternative interpretation’, Quaternary Science Reviews, 25,21-22, pp. 2692-2703.

2 comments:

hey, I like your blog, im doing a similar one myself. In this one in particular, you mentioned the importance of human overkill and climatic change in causing the extinction of mammoths. Both of these are credible causes and present the strongest arguments to explain such extinction patterns. However could hyper-disease have played a part? Rothschild & Laub (2006) support the hyper-disease theory by showing the extinction of a particular type of mammoth through human carried tuberculosis. Evidence of this was found in the disease being present in 52% of the 118 skeletons that were surveyed. Do you think this should be added to factors causing 'vanished giants'?

Thank you! I will check out your blog soon too. Yes in fact in my preliminary research I have come across the possible factor of hyper disease causing megafauna extinction, but the hypothesis does not seem as strong compared to human impacts and climate change. You have pointed out an interesting paper and I think having looked at the extinction dynamics on a number of continents now in relation to human factors and climate change, this might be the right time to do a post on other factors like hyper-disease!